I don't know if this will help, but it works well for me. The problem is that there are so many small things that don't get seen in the formula, like adding a foot for uphill putts, but not down, and subtracting a little extra for steep down. Anyway... This gets me real close on 13 and 14 greens.
13= #ft x .68 downhill / #ft x .76 for uphill
14= #ft x .62 downhill / #ft x .68 for uphill
13+ greens are the only time I use different multipliers for up and down. Anything slower than that, my numbers remain the same both up and down. If you play with that and it seems to be something that works for you, I would be glad to share all my numbers. I have them for every ".1" difference in green speeds. You can shoot me a message if you want them.
Note: The putter you have is a gr8 putter. It is the one I was using when I came up with all of my numbers.